ABSTRACT: The purpose of this study was to evaluate the sensitivity of a phosphorus (P) fractionation procedure on sampling practices and other sources of error in the characterization of the chemical status of P in noncalcareous sediments rich in humic compounds and iron (Fe). We investigated the effects of nitrogen gas (N2) shielding on the stability of various P fractions during sampling and various periods of sample storage on both oxic and reduced sediments. In addition, we studied storage-induced changes in preserved or unpreserved extracts from different fractionation steps. Sampling and storage conditions were critical factors in P fractionation studies. During sampling, sample storage, and extraction, the N2 atmosphere (atm) protected samples from oxidation and consequent changes in P forms, especially in the redox-sensitive form. Long-term storage without N2-shielding caused the most drastic changes in oxic sediment, whereas the reduced sediment protected with N2 experienced only minor changes. Extracts preserved with acid can be stored for at least 1 week without significant changes. The extraction results of P and other elements obtained with both oxic and reduced sediment samples supported the biogeochemical basis of the fractionation procedure, demonstrated by the behavior of Fe and Fe-bound P in sediment originating from and treated in different oxygen (O2) conditions. Despite a few shortcomings, with careful laboratory work and avoidance of sample oxidation during sampling and storage, the fractionation procedure and working practices used seem to provi